Some mechanical systems use bearings. Examples of such systems include but are not limited to subterranean drilling systems commonly used for oil and gas exploration and production. A subterranean drilling system typically circulates a fluid (for example, drilling “mud”, as it is known in the oil and gas industry) at high pressure through a down hole motor. Torque is generated by the fluid passing through the down hole motor's which is coupled to a mandrel constrained by bearings within a down hole bearing assembly. A distal end of the mandrel is coupled to a drill bit.
The wear resistance and/or load carrying capacity of the bearings may generally relevant to at least one of the performance of the subterranean drilling system, operating costs, and the length of time between service of the down hole motor assembly. Generally any improvement in the wear resistance and/or load carrying capacities of the bearing surfaces may be favourable.
For example, replacing the bearings now used in a down hole bearing assembly with bearings having better load bearing capacity and/or wear resistance may increase the maximum thrust load or weight-on-bit capacity of down hole motor assembly.
In another example, better bearing surfaces may improve directional drilling. The length of the bearing assembly is generally longer than desired for directional drilling. The minimum radius of a bend in a directionally drilled hole is limited to the distance between the end of the mandrel at which the drill bit is attached and a bend in the down hole assembly (“bit to bend distance”). One limitation to shortening bit to bend distances may be the length of the bearings. Improving at least one of the bearings limited load carrying capacity and wear resistance may enable the use of shorter bearings than now used in down hole assembly's that may reduce the bit to bend distance and consequently the minimum radius of a bend in a directionally drilled hole.
In yet another example, more wear resistant bearing surfaces may decrease the cost per hour of operation of the down hole assembly. The period between bearing replacements may be extended. Down time may be reduced. Wear of the bearings may result in a premature drop in the pressure of the fluid and increased radial and axial play, which are generally detrimental to at least one of drilling performance, drill bit life, the stability of the bearing assembly and the life of components therein. Threaded connections of subassemblies and even the mandrel shaft (that is, the central torque transmitting shaft) may experience increased flexural cyclic loading due to premature wear and play in the bearing system.
Longer life cycles of the bearing components may be sought to maintain peak performance, stability and life of the system as a whole.
Some bearings attempt to provide high wear resistance by the pre-placement of super hard monoliths, such as Thermally Stable Product (TSP) in a mold followed by the infiltration of metal powders surrounding them (a super hard material is a material with a Vickers hardness of greater than 40 GPa). This is time consuming and prone to filling defects beneath the monoliths. Disadvantageously, these monoliths are in very close proximity or touching the mold wall. When grinding of such a bearing commences, the super hard monoliths require grinding to provide a concentric and in-tolerance part which is generally difficult and expensive in view of the hardness of the super hard monoliths. Similarly, some bearings have Polycrystalline Diamond Compacts (PDCs) as bearing surfaces. These materials are generally backed by a supporting substrate such as cemented carbide. These are placed, bonded or joined to a steel substrate. The PDCs themselves are the order of several millimeters in diameter; i.e. 8-19 mm. To provide the required radii to form a bearing surface for a radial or angled thrust bearing requires significant grinding. Similar issues are encountered with silicon-bonded polycrystalline diamond monoliths.
Other mechanical systems that may be benefit from better bearings include, but are not limited to, rotary drill bits, generators, motors, aircraft, and locomotives.